1. Field of the Invention
The present invention relates to: a semiconductor switching element; and a semiconductor circuit apparatus, such as an inverter circuit and a motor circuit, which requires the flow of current using the semiconductor switching element not only in a forward direction but also in a reverse direction.
2. Description of the Related Art
Some of the semiconductor circuit apparatuses using this type of a semiconductor switching element (e.g., a motor circuit and an inverter circuit) require the flow of the current not only in the forward direction but also in the reverse direction.
When the current is intended to flow in a reverse bias direction in a conventional semiconductor switching element during an off-state, it is necessary to newly an incorporate diode at both ends of the semiconductor switching element, as shown in FIG. 5.
FIG. 5 is a circuit diagram showing an exemplary structure of an inverter circuit using the conventional semiconductor switching element.
In FIG. 5, for example, in the conventional inverter circuit 40, among four semiconductor switching elements 30a to 30d, two serial circuits, one consisting of the semiconductor switching elements 30a and 30b and the other consisting of the semiconductor switching elements 30a and 30d are connected in parallel between both ends of a power supply 41. Also, a capacitor 42 is connected between both ends of the power supply 41. An output circuit 43 (e.g., motor), which requires the flow of the current not only in the forward direction but also in the reverse direction, is connected between a connecting point of the semiconductor switching elements 30a and 30b making up the one serial circuit and a connecting point of the semiconductor switching elements 30c and 30d making up the other serial circuit.
With the structure described above, in the conventional inverter circuit 40 having a motor circuit structure, when the semiconductor switching elements 30a and 30d are on, and the semiconductor switching elements 30b and 30c are off, current flows from the semiconductor switching element 30a to the semiconductor switching element 30d via the output circuit 43 (e.g., motor). In this case, the current flows to the output circuit 43 (e.g., motor) in the forward direction.
When the semiconductor switching elements 30b and 30c are on, and the semiconductor switching elements 30a and 30d are off, current flows from the semiconductor switching element 30c to the semiconductor switching element 30b via the output circuit 43 (e.g., motor). In this case, the current flows to the output circuit 43 (e.g., motor) in the reverse direction.
Further, when the horizontal semiconductor switching elements 30a to 30d are off, no current flows to the output circuit 43 (e.g., motor), due to the pinch-off of the semiconductor switching elements 30a to 30d. 
Therefore, in the inverter circuit 40, in order to make the current flow in the reverse bias direction when the horizontal semiconductor switching elements 30a to 30d are off, diodes 44 for reverse bias operation are connected to the respective semiconductor switching elements 30a to 30d in parallel.
As an exemplary device structure of the semiconductor switching element, for example, Reference 1 proposes a device structure, in which an electrode, made of a high Schottky barrier material Ni and an electrode, made of a low Schottky barrier material Ti are provided on a SiC layer. It is reported that, with Ni electrode having the high Schottky barrier and Ti electrode having the low Schottky barrier, it is possible to realize a low-on resistance, and a pinch-off control by the high Schottky barrier.
Further, Reference 2 discloses a semiconductor apparatus in which a convex AlGaN layer is provided on an n-GaN layer in a diode and two types of anode electrodes having Schottky barriers of different heights are provided on the convex AlGaN layer. In other words, Reference 2 discloses a GaN semiconductor apparatus, in which a first anode electrode having a low Schottky barrier and a second anode electrode having a high Schottky barrier are provided on the convex portion of the surface of the n-GaN layer.
Further, Reference 3 discloses a semiconductor apparatus in which two types of anode electrodes having different widths from each other and having Schottky barriers of different heights are provided in a Schottky diode made of a GaN semiconductor. In other words, Reference 3 discloses the semiconductor apparatus in which a portion of a second anode electrode having a high Schottky barrier is provided so as to contact a first anode electrode having a low Schottky barrier, and the second anode electrode is also in contact with a semiconductor layer.
In addition, Reference 4 discloses a GaN semiconductor integrated circuit in which the Schottky diode described in Reference 3 and a transistor (FET; field-effect transistor) are integrated on the same substrate.
[Reference 1] “J. A. Cooper et al. “Recent Advances in Sic Power Devices.” Materials Science Forum vol. 264-268 (1998) pp. 895-900”
[Reference 2] Japanese Laid-Open Publication No. 2004-31896
[Reference 3] Japanese Laid-Open Publication No. 2005-317843
[Reference 4] Japanese Laid-Open Publication No. 2006-100645